The present invention generally relates to apparatus and methods for replacing mechanical aneroid barometers and, more particularly, to apparatus and methods of replacing mechanical aneroid barometers with sensors and a unique logic function.
In aircraft, cabin pressure is a fail safe critical system. If the system fails, people die. There is a constant inflow of air into the cabin. The outflow of that air is regulated by an outflow valve, the result of which is a regulation of cabin pressure. If the outflow valve were to fail/shut the air would accumulate and cabin pressure would be too high with the result that the cabin would puff up like a balloon. Conversely, if the outflow valve were to fail/open, there would be too little pressure in the cabin with the result that the cabin would decompress. The input information that governs whether the outflow valve should be closed or not is a reading of the existing cabin pressure. This job is performed by an auto channel controller, or by a monitor limit switch. Both may contain electronic pressure sensors that operate independently to monitor cabin pressure independently of one another.
In order to provide redundancy and back up for the fail safe critical system of cabin pressure, an aircraft cabin pressure control system may also include a mechanical aneroid switch, which serves to provide back up for the auto channel and monitor controllers. If cabin pressure is determined to be too low as measured by either of these two pressure sensors, or by the aneroid switch, then a relay called an altitude limit relay gets energized (a solenoid is turned on) and a switch is closed resulting in the outflow valve being closed.
In certain aircraft, such as the Gulfstream 150, the aneroid switch that provides redundancy for the auto channel and monitor limit controllers has not kept the failure rate of the cabin pressure system low enough.
As can be seen, there is a need for an apparatus and method that ensures that the altitude limit relay does not incorrectly activate at an unacceptable rate of failure and that the critical fail safe cabin pressure system does not fail.